The ACT Government's cull of more than 1,600 kangaroos in nature reserves was set to start today and last for the next three months.

But Animal Liberation spokeswoman Frankie Seymour says the group has launched a challenge in the ACT Civil and Administrative Tribunal (ACAT) and the cull has been suspended until another hearing next week.

"When a full hearing on whether or not there should be a stay, which if we achieve that, will then be in place until the full hearing of the merits of the case," she said.

A similar challenge was last year rejected by ACAT after it found the government was justified in bringing down kangaroo numbers to about 1.5 per hectare.

The government used research to demonstrate how rising kangaroo numbers were overgrazing and damaging sensitive flora.

Ms Seymour is confident Animal Liberation has a strong case.

"If we get a fair hearing, then I believe the cull will be stopped. If we get a fair hearing, but that of course is a big if."

She says the government still has not proved the kangaroo cull benefits the ecosystem.

"There is no data either baseline or data or evaluation, monitoring data to support any of the government's assertions about the benefits of the cull.

"There is no reason to believe it will even reduce the density of the kangaroos on the reserves until such time the kangaroos outside the reserves cease to exist."

The Territory and Municipal Services Directorate says eight grassland and woodland sites previously closed for the cull will reopen for public use and remain open until further notice.